1. Field of the Invention
The present invention relates to a portable information terminal apparatus such as a portable telephone, a PDA (Personal Digital Assistance), an HHT (Hand Held Terminal), a lap top PC (Personal Computer), and a voltage measurement apparatus installed therein; and in particular to a technique aiming at measuring a voltage value of a power supply-use battery with maintaining accuracy.
2. Description of the Related Art
A portable information terminal apparatus (simply “portable information terminal” hereinafter) is usually configured to be operable by a rechargeable battery. A capacity of the battery equipped on the portable information terminal is limited. Accordingly the majority of portable information terminals is equipped with the function (called a “battery monitor function” hereinafter) of displaying a remaining battery capacity or issuing a warning when the voltage value becomes no more than a certain value. An implementation of the function requires a measurement of a voltage of a battery. The measurement commonly uses an analog-to-digital converter (“ADC” hereinafter). This is because a voltage value can be handled by a digital value.
An ADC has an individual variation. There are many cases of using a configuration in which a voltage is applied to the ADC by dividing the voltage, and a resistance value of a resistor used therefor has a certain degree of variation. Due to this reason, some conventional voltage measurement apparatuses, such as the one noted in a patent document 3 for instance, among those which measure a voltage by using an ADC, are configured to have the capability of measuring a voltage highly accurately regardless of the above explained variation or error.
A conventional voltage measurement apparatus noted in the patent document 3 first connects an ADC to a reference power source which generates a high accuracy voltage, makes the ADC measure the voltage value and stores the measurement value in nonvolatile memory. It then connects an AC (alternate current) adaptor to the ADC, measures its voltage value, calculates a correction value from the two measurement values and stores the correction value in the nonvolatile memory, followed by calculating a voltage value of the AC adaptor by using a measurement result of the ADC and the correction value.
A use of the correction value for calculating a voltage value makes it possible to measure a voltage highly accurately. The conventional voltage measurement apparatus noted in the patent document 3 is configured to manually change over, by way of a connector, a power source connecting to the ADC. Due to this, work efficiency is low, that is, a cumbersome operation is necessary for a high accuracy measurement. This fact has required a technique for allowing a high accuracy measurement without involving a cumbersome operation.
In the meantime, there are some ADCs which measure a voltage value of a voltage as the subject of measurement by applying two voltages. Conventional voltage measurement apparatuses using such ADC include the one noted in patent documents 1 and 2 for example.
[Patent Document 1] Japanese Registered Patent No. 3609027 (i.e., Laid-Open Japanese Patent Application Publication No. 2002-221544)
[Patent Document 2] Laid-Open Japanese Patent Application Publication No. 2000-162251
[Patent Document 3] Laid-Open Japanese Patent Application Publication No. 2001-282368